Wax counterpart (4)
I believe the answer is:
wane
'wax counterpart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wane that I've seen before include "Opposite of wax" , "Atrophy" , "Dwindle" , "Get smaller" , "Anew (anag.)" .)